Scientifically, evening precedes the night. The night is also the whole of the dark where we sleep until the morning. You say good evening at the onset of the darkness because that is when it evening. You say good night to wish a pleasant rest of the night. The convention directly follows from the definition.
Yes, you can say night instead of goodnight. In fact, in many languages, the word for “goodnight” is simply a translation of the word “night.”. So there’s no reason why you couldn’t use the word “night” as a replacement for “goodnight” in English as well. This rule also applies to good afternoon.
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